Before running it in Toyota manual trans, check the specs. For example, G52 and W56 as well as many other Toyota transmissions require GL4/GL5. A gear lube which is rated as a GL5 only, has additives which may erode the brass syncos.
Read the label. Royal Purple 75w-90w, and Redline MT90 are two which have a GL4/GL5 rating. Diffs requiring a hypoid oil might be best served with a Redline oil as it needs oil which clings, and provides ample cushioning. I am partial to Redline in gear boxes. One can email Redline to get their recommendations. Synthetics are THE way to go in very cold climates. A standard 10w-30 may have a pour point at about 30-35 below zero. Mobil 1 would have a pour point at -58 below. Much easier cranking is noticable even above 0 degrees. The same goes for gear oil. With Dino oil in the box, it it feels like the brake is on, but no so with a synthetic.
There is a risk when changing over to synthetic in a high mileage motor, especially in which has not been served adequately, as sluge and varnish build up on worn and hard seals can help prevent seepage. A synthetic oil is highly detergent and can remove that build up. This may not as an important factor if the motor was run a Dino oil which itself was highly detergent and/or change regularly. That said, my personal experience has been good.
I totally lucked out last year on a purchase of 85' EFI 2WD extra cab auto pickup with only 53K. Cherry! They are still out there! The 80 year old fellow couldn't drive anymore. It had been poorly serviced and sat in the garage for years. The diff oil had actually separated and the seals leaked. All other was good, but the timing chain rattled excessively on start up. Mobil 1 apparently cleaned out the timing chain tensioner, and she rattles no more. And just as important, eventho I expected it to leak like crazy, there has been no leaks and does not burn it.